Electrophoretic displays, such as those that may be used in e-reader devices are displays based on the electrophoresis phenomenon influencing charged pigment particles suspended in a dielectric solvent. The pigment particles may be of a size of about 1-2 microns in diameter, carry a charge, and are able to migrate within the solvent under the influence of externally applied charges from adjacent electrode plates.
An electrophoretic fluid may have at least one type of charged pigment particles dispersed in the solvent or solvent mixture, and the electrophoretic fluid may be pigmented with a contrasting color to the color of the particles, for example, white particles in a colorless or clear solvent. Upon application of a charge to the electrode plates, the charged particles may be caused to migrate towards or away from the plates, by attraction to a plate of opposite charge, and repulsion away from a plate of similar charge. In this manner, the color showing at one surface may be either the color of the solvent if the particles are attracted away from that surface, or may be the color of the pigment particles if the particles are attracted to that surface. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color.
Alternatively, an electrophoretic fluid may have two types of pigment particles of contrasting colors (for example, white and black) and carrying opposite charges, dispersed in a clear solvent or solvent mixture. Upon application of a voltage difference between the two electrode plates, the two types of pigment particles may move to opposite ends (top or bottom) in a display cell. Thus, one or the other of the colors of the two types of pigment particles would be visible at the viewing side of the display cell.
The pigment particles may be ionic or ionizable microparticles composed of a polymer encapsulating white, black or otherwise colored molecules. The particles may be formed from a non-covalent bonding of polymer matrix to encapsulated dye molecules, and because of the non-covalent bonding may lose color over time, or may be broken down by radiant energy and no longer function as designed.
For electrophoretic displays, there remains a need for charged pigment particles which have improved color-fastness and photostability.